* lib/uchar.in.h (mbrtoc16): New declaration.
* lib/mbrtoc16.c: New file.
* m4/mbrtoc16.m4: New file, based on m4/mbrtoc32.m4.
* modules/mbrtoc16: New file.
* m4/uchar_h.m4 (gl_UCHAR_H): Test whether mbrtoc16 is declared.
(gl_UCHAR_H_REQUIRE_DEFAULTS): Initialize GNULIB_MBRTOC16.
(gl_UCHAR_H_DEFAULTS): Initialize HAVE_MBRTOC16, REPLACE_MBRTOC16.
* modules/uchar (Makefile.am): Substitute GNULIB_MBRTOC16,
HAVE_MBRTOC16, REPLACE_MBRTOC16.
* doc/posix-functions/mbrtoc16.texi: Mention the mbrtoc16 module and the
mbsinit related limitation.
* doc/posix-functions/mbsinit.texi: Mention the mbrtoc16 related
limitation.